Temperature optimizer control apparatus and method

ABSTRACT

Apparatus and method for controlling compressor operation in conjunction with sensing the lowest temperature operating evaporator coil, or area serviced thereby, of a commonly-piped cooling system. A thermoelectric sensor voltage output is compared with a set-point voltage by an integrator, the output of which is applied to control the pressure for the system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of pending application Ser. No.458,914 filed Jan. 18, 1983, now abandoned, which is acontinuation-in-part of Ser. No. 257,113 filed Apr. 24, 1981, which wasa continuation of application Ser. No. 062,525, filed July 31, 1979, nowabandoned. Applications referred to herein were filed by the sameapplicant as applicant in the present application.

FIELD OF THE INVENTION

This invention relates to increasing the efficiency of cooling orrefrigeration systems, especially for those systems employingcommonly-piped evaporator coils where at least one set of coils islocated for temperature controlling one compartmentalized ornon-compartmentalized area and another set of coils is located fortemperature controlling another compartmentalized ornon-compartmentalized area.

BACKGROUND OF THE INVENTION

Cooling systems generally comprise a condenser coil, a receiving vesselfor the condensed liquid from the condenser coil, an expansion valve, anevaporator coil, and a compressor. The compressor is connected to thecondensor coil. In addition, such a cooling system includes a defrostmechanism for the evaporator coil since the moisture that tends toaccumulate thereon turns to ice during operation and would, in time,build up to a degree that would make the cooling system almost totallyinefficient, if not inoperable.

Improvements over the simplest cooling system briefly described abovehave included using multiple compressors, rather than only one, andalternating their use in accordance with demand so as to use only enoughcompressor capacity sufficient for the demand and so as to minimize wearon the compressors. Such an energy efficient system is shown anddescribed in co-pending U.S. patent application Ser. No. 257,113, filedApr. 24, 1981, by the same inventor as the present application. Thesystem of application Ser. No. 257,113 represents a technique forachieving more efficient operation through the use of a single highestfixed "cut in" and a single highest fixed "cut out" operating suctionline pressure while insuring adequate temperatures in the refrigeratedspaces served by the refrigeration system.

In the past, the cycling of stages of a multiple-stage refrigeration orcooling system has been principally accomplished by setting each stageat a successfully lower "cut in" and "cut out" pressure of therefrigerating fluid flowing in the suction line from the evaporator coilto the compressor(s) or cooling stages. The use of successfully lower"cut in" and "cut out" pressure ranges for each cooling stage results inan average pressure which is lower than the mean pressure of thepressure differential between the "cut in" and "cut out" pressures ofthe highest stage. Various mechanical and electromechanical systems havebeen devised to attempt to solve this problem, primarily utilizing thesuccessively lower pressure ranges for each successive cooling stage asdescribed above.

The invention of application Ser. No. 257,113, briefly stated, involvesan apparatus and method of controlling the capacity of a multiplecompressor refrigeration system. The invention first establishes aselected cooling stage "cut-in" and "cut-out" suction line pressure forthe system. Secondly, the system determines when the suction linepressure is equal to either of these pressures, and based on thisdetermination, controls the selection of the appropriate capacity forthe system to achieve an average operating suction line pressure betweenthe selected single highest "cut-in" and single highest "cut-out"pressures.

The system described herein is another way or technique for increasingthe efficiency of many cooling systems, and may be used independently orwith the system described herein and in application Ser. No. 257,113 toachieve a cummulative system efficiency increase.

It is not unusual for the same compressor (or system of compressors) tobe commonly piped in a cooling system which employs more than oneevaporator coil. The reason for this is that the capacity of the systemis sufficiently designed for cooling a given overall area; however, someof this area is compartmentalized separately from the rest of this area.Moreover, it is commonly desirable to reduce the temperature in one areato a lower temperature than is required for another. By example then, ifone area is smaller in size than another and both are cooled in the samemanner by similar evaporator coils, then the smaller area will be cooledat a lower temperature than the larger of the areas.

Rather than have two completely different systems when there are two ormore separate areas to cool, such systems having duplicate compressors,duplicate condensor coils, duplicate condensor fluid vessels, and thelike, it is less expensive to have one system with these commoncomponents commonly-piped, usually in parallel rather than in-line, withseparate evaporator coils only. Although separate coils are commonlyemployed in respective separate compartments, it is also usual forseparate coils to be used for a common large area, where one coil isused, for example, in one room and another coil is used in a secondroom, one room being kept at a lower temperature than the other.

It is also known that compressors can be operated to raise the pressurein a cooling system to warm the area cooled by an evaporator coil aswell as to lower the pressure to cool the area cooled by an evaporatorcoil. The system to be described hereinafter includes this capability.

Therefore, it is an object of the present invention to provide animprovement for optimizing the pressure control of a cooling systemhaving at least two commonly-piped evaporator coils.

It is another object of the present invention to provide an improvementfor providing pressure control to a cooling system wherein the furtheraway from a norm temperature an area may be, e.g. a temperature setpoint, the more rapid will be the rate of adjustment of pressure.

SUMMARY OF THE INVENTION

The invention herein disclosed is an apparatus and method forcontrolling compressor operation of a cooling system having a pluralityof commonly-piped evaporator coils. One of the coils, or the area cooledby one of the coils, desirably operates at a temperature level lowerthan the others. A thermoelectric probe senses this temperature anddevelops a voltage proportional thereto. A voltage reference means isadjustably set to have an output voltage which is equal to the voltagethat the thermoelectric probe produces when the monitored coil or areais at the desired temperature.

The voltage outputs of the thermoelectric probe and the reference meansare applied as inputs to an integrator, the output of which is connectedto a pressure adjustment. When the integrator registers that thedetected or sensed temperature is too cold, the setting is raised whichraises the pressure in the cooling system and raises the temperature ofthe coldest area. When the integrator registers that the detected orsensed temperature is too warm, on the other hand, the pressureadjustment is lowered thereby lowering the pressure in the coolingsystem and, hence, lowering the temperature.

When the coil or compartment being monitored is defrosted, a switch isoperated to short the inputs of the integrator and thereby toeffectively prevent the output from changing.

Because of the integrator, which is effectively a comparator with an RCtime constant, the output therefrom integrates the difference in itsinputs. In other words, the output will follow an integration curve and,therefore, accentuate large differences to quickly bring the monitoredcondition to the desirable temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features, advantages andobjects of the invention, as well as others which will become apparent,are attained and can be understood in detail, more particulardescription of the invention briefly summarized above may be had byreference to the embodiment thereof which is illustrated in thedrawings, which drawings form a part of this specification. It is to benoted, however, that the appended drawings illustrate only a preferredembodiment of the invention and are therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

In the Drawings

FIG. 1 is a simplified mechanical/electrical block and schematic diagramof a cooling system employing multiple compressors and multipleevaporator coils, all of which are controlled by a controlling apparatusin accordance with the present invention;

FIG. 2 is a block diagram of the electrical portion of the controllingaparatus shown in FIG. 1;

FIG. 3 is a simplified schematic diagram of a portion of the controllerapparatus shown in FIG. 1 which generates pressure cut-in and cut-outcomparator control signals;

FIG. 4 is a simplified mechanical/electrical block and schematic diagramof a cooling system employing multiple compressors which are controlledby controlling apparatus in accordance with the present invention tocontrol the cooling from a single evaporator coil;

FIG. 5 is a simplified block diagram of one embodiment of a controlcircuit responsive to the suction line pressure, for controlling theselection of compressors and thereby controlling the suction linepressure, of the cooling system shown in FIG. 4; and

FIG. 6 is a graphical representation of the refrigerating fluid suctionline pressure versus time for a multiple-stage refrigeration or coolingsystem operating within the parameters of the capacity control methodand apparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 4, the refrigeration system capacity controllercircuit 36 is shown disposed in a multiple-stage refrigeration orcooling system 20' consisting of a plurality of parallel stagedrefrigerant compressors 12', 14', 16', and 18' for dischargingcompressed pressurized refrigerant vapor through discharge line 22' to acondenser coil 25' where the pressurized refrigerant vapor is condensedto a liquid and then delivered to a receiver vessel 26'. From thereceiver 26', the liquid refrigerant flows through line 28' and throughan expansion device or valve 30', typically a mechanical expansion valveresponding to the temperature in suction line 38 as sensed bytemperature sensing device 32. The temperature signal from sensor 32 isapplied to valve 30' through conductor 33 to initiate the expansionvalve action.

The liquid refrigerant is injected through expansion device 30' into theevaporator coil 35 where the liquid refrigerant, encountering the lowpressure of the evaporator coil, boils and evaporates thus absorbingheat from the evaporator coil. The hot vaporized refrigerant from theevaporator coil is drawn through suction line 38 to the inlet ports ofthe multiple compressors 12'-18'. The number of parallel compressors tobe staged in the system varies according to the refrigerating or coolingsystem load. In FIG. 4, three compressors are shown as 12', 14' and 16',and an "Nth" compressor represented in dotted lines by compressor 18'.

A pressure transducer 40' is attached to the suction line 38 anddetermines the refrigerant vapor pressure within suction line 38 andgenerates an electrical signal representative of the measured suctionline pressure. The signal is applied through conductor 42' as an inputto the capacity controller circuit 10', which will be hereinafterdescribed in greater detail. The output of the controller circuit 36 isa plurality of outputs corresponding to the number of the plurality ofcooling stages or parallel compressors staged in the system.Accordingly, there are a corresponding "N" number of outputs from thecapacity controller circuit 36 labelled 1, 2, 3 and N.

The controller circuit output 1 is applied through conductor 56 to thecoil of a relay 44' which controls relay switch contacts 45 for applyingAC power via conductors 52 and 54 to the first compressor 12' forenergizing the compressor when it is desired to cut the compressor intothe system. Similarly, the 2, 3 and N outputs of the capacity controllercircuit are applied through conductors 58, 60 and 62, respectively, tothe coils of relays 46', 48' and 50', respectively, for successivelyclosing switches 47', 49 and 51, respectively, for successively applyingAC electrical power to the 2, 3 and N compressors, respectively, foreither turning on or turning off the compressors in a staged sequence.

Referring now to FIGS. 4, 5 and 6, the operation of the capacitycontroller circuit 36 will be described in greater detail. The pressuredetecting means or transducer 40' is shown sealingly inserted into therefrigerant vapor flow 65 in suction line tubing 38. Pressure transducer40' may be any conventional pressure detecting means for generating anelectrical signal representative of the pressure within line 38. Thepressure signal from transducer 40' is applied through conductors 42'and 70 to the positive input of a comparator circuit 74, and throughconductors 42' and 72 to the negative input of a second comparatorcircuit 76. To set a predetermined "cut-in" pressure for the system, avoltage potential is applied through a voltage varying means, such as apotentiometer 75, to the negative input of the comparator circuit 74.Similarly, a voltage is applied through a voltage varying means, such asa potentiometer 77, to the positive input of the comparator circuit 76to set a predetermined "cut-out" pressure for the system. As will bepointed out below with respect to the cut-in and cut-out comparatorcontrol voltages generated in accordance with the present invention (SeeFIG. 3), the "cut-in" control voltage for the negative input ofcomparator 74 and the "cut-out" control voltage for the positive inputof comparator 76 to control a multiple compressor, multiple evaporatorcoil cooling system could be generated in response to the temperature ofthe coldest compartment to be cooled thereby to dynamically achieve thehighest suction line pressure operating condition while meeting thecooling requirements of the coldest compartment.

Comparator circuit 74 compares the predetermined cooling system "cut-in"pressure (set by potentiometer 75) against the suction line pressurecontinuously detected by pressure transducer 40' and produces a "cut-in"electrical signal when the measured pressure exceeds the predetermined"cut-in" pressure. Comparator circuit 76 compares the predeterminedcooling system "cut-in" pressure (set by potentiometer 77) against thepressure continuously detected by the pressure transducer 40' andproduces a "cut-out" electrical signal when the detected system pressureexceeds the predetermined "cut-out" pressure. The combination oftransducer 40', potentiometers 75 and 77 for establishing systempressure "cut-in" or "cut-out" pressure levels and the comparators 74and 76 also comprise detection means 64 for establishing a selectedcooling stage "cut-in" or "cut-out" pressure and determining when thoseestablished pressures have been reached and providing an output signalin response thereto, i.e., output of comparators 74 or 76.

The "cut-in" signal of comparator 74 is applied through conductor 78 toa timer circuit 82' which generates an output electrical control signalafter a first predetermined minimum time delay. The delayed time outputcontrol signal is applied through conductor 86' as an input to a counter90. Counter 90 is a conventional counter circuit that generates asuccessive plurality of outputs 1 to N corresponding to the number ofstaged parallel compressors in the system. Each delayed control signalreceived from timer 82' by counter 90 will cause a counter output signalto appear at one of the series of successive counter outputs 1, 2, 3 orN in repetitive succession, and are applied via a conductor 95 as a"set" input to a latch circuit 94.

The latch circuit 94 is of conventional solid-state design and generatesa series of repeatable successive "cut-in" electrical control signals atoutputs 1, 2, 3 and N, to be applied through conductors 56, 58, 60 and62, respectively, to a series of repeatable successive compressorcontrol relays 44', 46', 48' and 50', respectively, in response tosuccessive "set" input signals received via conductors 95 from the 1, 2,3 or N outputs of counter 90. For example, if the counter 90 has anoutput signal at output 1 applied through a conductor 95 as a "set"input to latch circuit 94, latch circuit 94 will generate a controlsignal output voltage at its 1 output (conductor 56). Similarly, outputsignals from counter 90 appearing in a series successively at 2, 3 and Nare applied through conductors 95 as repeatably successive "set" inputsto latch circuit 94, thereby generating "cut-in" or turn on controlsignals appearing at outputs 2, 3 and N (conductors 58, 60, and 62,respectively). The electrical control signals are voltages appliedthrough conductors 56, 58, 60 and 62, respectively to relays 44', 46',48' and 50', respectively, as hereinabove described for successivelyenergizing the relays 44', 46', 48' and 50', respectively, andsuccessively energizing or turning on one of the multiple compressors12', 14', 16' and 18', respectively.

Similarly, the "cut-out" output signal of comparator 76 is appliedthrough conductor 80' to a timer 84'. Timer circuit 84' generates adelayed "cut-out" electrical control signal after a predetermined timedelay. The control signal is applied through conductor 88 as an input toanother counter circuit 92. Counter 92 may be identical to the counter90 hereinabove described. Each successive delayed "cut-out" controlsignal received from timer circuit 84' generates one of a series ofrepeatable successive electrical signals at counter 92 outputs 1, 2, 3and N, which are applied thorugh conductors 93 as "reset" inputs tolatch circuit 94. Receipt of the successive series of delayed "cut-out"control signals from counter 92 causes the latch circuit 94 to be resetin the succession in which the counter signals are received.

For example, upon receipt of a counter 92 output 1 signal appliedthrough conductor 93 as a "reset" input to latch circuit 94, the latch94 output at 1 will be reset and no voltage will appear on conductor 56thus de-energizing relay 44', opening relay switch contacts 45 and"cutting-out" the first compressor 12', which has run the longest.Accordingly, successive counter 92 signals received from outputs 2, 3and N as "reset" inputs to latch circuit 94 will successively reset thelatch circuit and remove the latch circuit voltage outputs appearing atlines 2, 3 and N (conductors 58, 60 and 62, respectively), for"cutting-out" compressors 14', 16' and 18' in succession.

In addition, the latch circuit outputs 1, 2, 3 and N (conductors 56, 58,60 and 62, respectively) are also connected by conductors 99 as inputsto a conventional AND gate 96. When all of the latch circuit outputs 1,2, 3 and N have positive output voltages appearing thereon, the AND gate96 generates an output signal applied through conductor 97 to counter 90to disable counter 90 at its last count and prevent further delayed"cut-in" electrical signals received from timer 82' from generatingfurther counter 90 output electrical signals for application to thelatch circuit 94. Similarly, the latch circuit outputs 1, 2, 3, and Nare also connected by means of conductors 101 as inputs to aconventional NOR gate 98. NOR gate 98 will generate an electrical outputsignal to be applied through conductor 103 to disable counter 92 whenall of the latch circuit outputs 1, 2, 3 and N have been reset and thereare no output voltage signals present thereon. The electrical signalreceived from NOR gate 98 disables counter 92 and prevents any further"cut-out" delayed signals received from timer 84' from triggering anyfurther counter 92 output signals to be applied as reset inputs to latchcircuit 94.

The operation of the capacity controlling circuit 10' can now further bedescribed with reference to FIGS. 4, 5 and 6. The graph of FIG. 6depicts the system suction line refrigeration fluid pressure vs. timeand is represented by pressure trace 121. The selected suction line"cut-in" pressure represented by line 120 is set for the system bypotentiometer 75 (or possibly the "cut-in" comparator voltage from FIG.3) as one input to the comparator 74, as hereinabove described. Theselected "cut-out" pressure represented by line 130 is set bypotentiometer 77 (or possibly the "cut-out" comparator voltage from FIG.3) as one input to comparator 76, as hereinabove described. The desiredsystem suction line pressure range ΔP has been selected for optimumefficiency of the system. The timer 82', as hereinabove described,establishes a predetermined delay time which is represented by the timeinterval ΔT, and the delay time established by timer circuit 84' isrepresented by the shorter time interval Δt. Assuming that compressors 1and 2 are operating in the system within the ΔP established by "cut-in"pressure (120) and the "cut-out" pressure (130), if the refrigeratorload increases, then the suction line pressure will rise. If the load isheavy enough, the pressure (121) will rise until it exceeds thepredetermined value established by potentiometer 75 (line 120) at point122 and comparator 74 will generate an electrical "cut-in" signal to beapplied to the timer 82.

The comparator signal output occurs at point 122 which is the point atwhich the suction line pressure 121 rises above or exceeds thepredetermined cut-in system pressure and establishes the beginning ofthe delay time ΔT or timer 82'. The suction pressure may continue torise as shown in FIG. 4 until timer 82' generates its delayed "cut-in"electrical control signal which is applied to counter 90, and sincecompressors 1 and 2 are already operating, counter 90 will generate anoutput signal at output 3 which is then applied through a conductor 95as a "set" input to latch circuit 94. Receipt of the delayed "cut-in"signal from output 3 of timer 82' by the latch circuit 94 causes apositive voltage to appear at latch output 3 (conductor 60) which isapplied to relay 48' for energizing the relay, closing relay switch 49and "cutting-in" the third compressor 16', which has been "turned off"the longest time period. The end of the predetermined time delay, ΔT,established by timer 82', occurs at point 124, and the third compressoror cooling stage now in the system adds cooling capacity and returns thepressure to the operating pressure differential ΔP range established bypressures 120 and 130. In the event that the combined operating capacityof compressors 12', 14' and 16' is still insufficient for the load, thesuction pressure will not drop below the cut-in pressure (120) andcut-in timer 82' will generate another delayed "cut-in" signal to cut-inanother compressor stage, up to the Nth stage to match the load demand.

In the event the suction pressure (121) declines because ofover-capacity in the system, and falls below the predetermined "cut-out"pressure represented by line 130, then comparator 76 will generate a"cut-out" signal applied to the timer 84' occurring at point 126, whichbegins the established time delay Δt. When the predetermined time delayΔt has elapsed, timer 84' generates a "cut-out" electrical controlsignal applied through conductor 88 as an input to the counter 92. Thecounter 92, in response to the received signal, will generate an outputsignal on line 1 applied through a conductor 93 to latch circuit 94 as a"reset" input. The counter reset signal applied to latch circuit 94 will"reset" output line 1 of the latch circuit, thereby removing thepositive voltage output at conductor 56 and de-energizing relay 44',opening switch contacts 45 and "cutting-out" compressor 12' (which hasbeen operating the longest time period) from the system, as reflected inFIG. 6 at 128, the end of the delay Δt and the point where the suctionpressure again begins to increase. When compressor 12' is "cut-out" ofthe system, the suction line pressure begins to increase until itreturns to the desired operating range between the pressuredifferentials 120 and 130. Similarly, in the event that the combinedoperating capacity of the compressors or cooling stages still exceedsthe load, another "cut-out" signal will be generated by timer 84' to"cut-out" another compressor stage until the operating stage capacitymatches the system load.

In this way the multiple staged compressors can be "cut-in" or "cut-out"of the system to increase or decrease refrigeration capacity dependingon the system refrigeration load by a discrete combination of compressorstages, thus matching as closely as possible the available compressorstage capacity with the system load. The compressor that has operatedthe longest will always be the first to be "cut-out" when the systemcapacity needs to be diminished, and the compressor that has notoperated the longest will be the next to be "cut-in" when the systemcapacity needs to be increased.

The timers 82' and 84' and counters 90 and 92 "remember" the length oftheir respective time delays, ΔT and Δt. For example, referring to FIGS.4, 5, and 6, if ΔT is five (5) minutes, and Δt is five (5) seconds, ifsuction pressure 121 rises above the "cut-in" pressure (120) at point122, the five (5) minute ΔT period begins. However, if suction pressuretrace 121 had dropped back below "cut-in" pressure 120 after only two(2) minutes had elapsed (or before reaching poing 124), the "cut-in"signal from comparator 74 will cease, disabling timer 82'. Similarly, ifsuction pressure trace 121 falls below the "cut-out" pressure 130 as atpoint 126, the five (5) second Δt period begins. However, if the suctionpressure (121) increases and rises above "cut-out" pressure (130) afteronly three (3) seconds, the "cut-out" signal from comparator 76 willcease and disable timer 84', and no delayed "cut-out" signal will besent to counter 92. Accordingly, no delayed "cut-in" signal will beaddressed to counter 90. Therefore, no additional compressor or coolingstage will be "cut-in", but the next time the suction pressure 121exceeds the "cut-in" pressure, timer 82' will again be energized andwill produce a delayed "cut-in" signal after only three (3) minutes (thebalance of ΔT left over from the last ΔT period) and "cut-in" or turn onthe next compressor or cooling stage of the system. Similarly, the nexttime the pressure trace 121 decreases to fall below the "cut-out"pressure, timer 84' will again be enabled and will produce a delayed"cut-out" signal after only two (2) seconds (the balance of Δt left overfrom the last Δt period) and "cut-out" or turn off the compressor whichhas run the longest.

As above described, it will be evident that controller circuit 10' will"cut-in" or "cut-out" the next compressor or compressor stage as abovedescribed until the combination of stages has an operating capacityclosest to matching the system load, i.e., causing the system suctionpressure to return to the previously established ΔP range as hereinabovedescribed and shown in FIG. 6. For example, if the compressors 12', 14'and 16' are the only compressors in the system, then the capacitycontroller 10' will select and provide increased or decreased compressorcapacity in combinations to most closely match the load demand. Thevarious combinations of compressors 12', 14' and 16' will be 12' alone,12'-14', 12'-14'-16', 12'-16', 14'-16', 14' alone or 16' alone.

Similarly, if the compressors 12', 14' and 16' are unequal in capacity,and rated at 1, 2 and 4 horsepower (HP), respectively, then it has beenfound that the capacity controller 36 will select and provide increasedor decreased compressor horsepower capacity in discrete increments orcombinations to match the load demand. Assuming the above describedratings of 1, 2 and 4 HP for compressors 12', 14' and 16', then thevarious possible combinations of those compressors will providecapacities of 1, 2, 3, 4, 5, 6, and 7 HP in response to changing loaddemand. The number of combinations for multiple compressors whether ofequal or unequal capacity will always be larger than the number ofcompressors or compressor stages in the system. It will be furtherevident from the above described operation and drawings showing multiplecompressors 12', 14', 16', and 18' (1, 2, 3 to N number of compressors)that any number of parallel compressors can be controlled, such as asystem of two (compressors 12' and 14'), a system of three (compressors12', 14' and 16') or a system of N number (compressors 12', 14', 16'-N).

The refrigeration capacity control invention herein disclosed may alsobe utilized in controlling multiple stage refrigeration of coolingsystems having multicylinder compressors that are staged by controllingthe compression of a plurality of compressor cylinders usingconventional control valves by having controller 36 outputs control theutilization of the cooling stages by controlling the cylinders used bythe compressors in the system. In addition, it is important tounderstand that while the system above described in FIGS. 4, 5, and 6uses a separate time delay after determination of the reaching of theestablished "cut-in" or "cut-out" pressures, only a single time delay isnecessary to enable selection of successive cooling stages utilizing asingle selected "cut-in" system pressure and a single selected "cut-out"system pressure. For instance, in FIG. 5, the output of "cut-out"comparator 76 could be applied to timer 82' and utilize a single delaytime for both "cut-in" and "cut-out" determinations. Further, the ΔPdifferential between "cut-in" pressure 120 (FIG. 6) and "cut-out"pressure 130 may be large or small, depending on the system design andthe best system operating pressure. In certain systems, the ΔP could beset at zero, with the "cut-in" and "cut-out" pressures being establishedat the same value.

Turning now to FIG. 1, a multiple compressor, multiple evaporator coilcooling or refrigeration system suitable for operation in conjunctionwith the present invention is illustrated. The multiple compressorsystem of FIG. 1 can also be operated in accordance with the abovedescribed principles of system compressor pressure control.

As shown in FIG. 1, a plurality of parallelstaged refrigerantcompressors 10, 12 and 14 are each separately connected for dischargingcompressed pressurized refrigerant vapor through common discharge line16 to a condenser coil 18. The pressurized refrigerant vapor iscondensed to a liquid in condenser coil 18 and delivered to a receivingvessel 20. From this receiving vessel 20, the liquid refrigerant flowsthrough a common line 22 and then through line 24 to an expansion deviceor valve 26 for operation in conjunction with a first evaporator coil27. The liquid refrigerant also flows from common line 22 through line23 to an expansion device or valve 25 for operation in conjunction witha second evaporator coil 29.

The liquid refrigerant is injected through the respective expansionvalves 26 and 25 into their respective evaporator coils 27 and 29, wherethe liquid refrigerant, encountering the low pressure of the evaporatorcoils, boils and evaporates, thus absorbing heat from the coils andcooling the surrounding respective areas 30 and 32.

The outputs of evaporator coils 27 and 29 are connected together and tocommon suction line 34. The vaporized refrigerant from the evaporatorcoils is drawn through suction line 34 where it is then delivered to therespective inlet ports of multiple compressors 10, 12 and 14. Thepressure transducer 40' described with respect to the multiplecompressor, single evaporator coil system shown in FIG. 4, is shown inFIG. 1 coupled to suction line 34 for measuring the suction linepressure, i.e., the compressor pressure operation.

Although it is beneficial to use the inventive optimizing temperaturecontrol apparatus hereinafter described more in detail with a simplifiedsystem using only a single compressor, it is desirably used inconjunction with the illustrated multiple compressor system forcumulative results therewith. That is, the substantial advantages of themultiple compressor system operated in accordance with the abovedescribed compressor pressure control operation are further enhanced byoptimizing in a refrigeration system having multiple evaporator coilsthe compressor pressure control range to achieve the minimum systemcompressor capacity required to maintain the highest suction linepressure thereby to maintain the desired temperature of the coldestcompartment to be cooled.

The compressors shown in FIG. 1 may be cycled using suitable cut-in andcut-out suction line pressure control signals developed by potentiometer75 and 77, respectively, in the manner above described. That is, thedesired cut-in pressure is set by potentiometer 75 and compared incomparator 74 to the actual measured suction line pressure from pressuretransducer 40'. If the measured pressure exceeds the setting, a cut-insignal is generated to timer 82'. Similarly, potentiometer 77 sets thecut-out pressure, and when the measured pressure is less than thecut-out pressure, a cut-out signal is generated to timer 84'.

In accordance with the present invention, a circuit is disclosed foroptimizing the compressor pressure operation in a cooling system havingmultiple compressor and multiple evaporator coils to obtain the highestpossible suction line pressure, i.e., the lowest compressor capacity,that will satisfy the lowest desired temperature required by any of theevaporator coils. A thermoelectric probe or sensing device 44 ispositioned with respect to the evaporator coil or the area served by theevaporator coil that operated at the lowest temperature to generate anoutput voltage representative of the temperature in such compartment. InFIG. 1, it is assumed that coil 27, or area 30, is operated at atemperature lower than coil 29, or area 32. Devices known asthermocouples are well known examples of suitable thermoelectric probes.

For simplicity, area 32 is assumed to be warmer than area 30 even thoughsubstantially identical coils 27 and 29 are shown in FIG. 1. In anactual system, the maintaining of coil 29 warmer with respect to coil 27is usually accomplished by the presence of an evaporator pressureregulator valve 31 in the warmer of the two coils. Such a valve 31effectively restricts the line opening and raises the pressure, andhence the temperature, in coil 29 vis-a-vis the temperature of coil 27.A suitable thermocouple is used as the temperature sensing element 44for sensing the temperature of the coldest area to be maintained.

In addition to whatever other initiating signals are produced bycompressor controlled circuit 36, the signal produced by temperaturesensor 44 is also used as an additional control signal, which can bebetter understood by reference to FIG. 2. Referring to FIG. 2, a voltageset point device 46 establishes a reference voltage output which isequal in value to the output from temperature sensor 44 when themonitored coil or area is at the desired norm temperature. The outputsfrom temperature sensor 44 and from voltage set points 46 and thenconnected as the respective inputs to an integrator 48.

Integrator 48 is a comparison type of adjustment means that produces anintegrated output voltage dependent on the difference in the appliedinputs. Whenever the output from temperature sensor 44 indicates thatthe monitored conditions in the area 30 are warmer than the referencesetting, then an adjustment output voltage is produced on the output ofintegrator 48 for reducing the suction line pressure and hence thetemperature being monitored. On the other hand, whenever the output fromtemperature sensor 44 indicates that the monitored conditions are colderthan the reference, then an adjustment output voltage is produced forincreasing the suction line pressure and raising the temperature beingmonitored. The output of integrator 48 is an integrated voltage, eitherpositive or negative depending on which of the two input voltages ispositive with respect to the other.

In accordance with operation well-known in the art, defrosters 28 and40, respectively, are operated in suitable fashion for defrosting,respectively, coils 27 and 29. A defrost control 42, which may include atimer or frost sensor or similar device, activates and deactivates therespective defrosters. When defroster 28 is operated by defrost control42, a signal is also produced for operating switch 47 to short togetherthe inputs to integrator 48. This asures that there is no further changein the output from integrator 48 during this time. When the defrostcycle is over and defroster 28 is deactivated, then switch 47 is allowedto open to place the circuit in operation to continue as before.

It is well known by those skilled in the art that an integrator is ananalog memory device, it remembers what has happened in the past. If theinput voltage to be integrated is zero volts, there will be no change inthe integrator output voltage. That is, whatever voltage is on theoutput of the integrator, that voltage will remain unchanged as long asthere is no further voltage to be integrated. Further, the rate at whichan integrator changes its output voltage is dependent upon the magnitudeof the voltage to be integrated. Thus, when the defrost lockout switch47 is closed, there will be no further change in the signal, TEMP. INT.INPUT, at the output of the integrator 48. Similarly, when defrostlockout switch 47 is open, the magnitude of the difference between thevoltage generated by the temperature probe 44 and the voltage set point46, i.e., the norm temperature desired for area 30, will determine therate at which TEMP. INT. INPUT changes and by how much.

In accordance with the present invention, this integrated comparisonbetween the measured temperature of area 30 and the desired normtemperature for that area is used to dynamically control the compressorpressure operation to optimize the operating suction line pressure atthe highest pressure that will maintain the cooling in the coldestcompartment, i.e., compartment 30. As described above with respect tothe multiple compressor refrigeration system shown in FIGS. 4 and 5, thecompressor control circuit 10' (See FIG. 5) (compressor control circuit36 of the system of FIG. 1) produces a fixed "cut-in" and fixed"cut-out" suction line operating pressure (see, for example, FIG. 6).The pressure of the suction line is monitored and compared to these tworeference pressure settings by comparators 74 and 76, respectively, toproduce control signals to adjust the compressor capacity to effectcontrol of the suction line pressure to an average value between thecut-in and cut-out pressure settings.

Now turning to FIG. 3, there is shown a circuit responsive to thetemperature in the coldest compartment to be maintained, for generatingcut-in and cut-out voltages for use in conjunction with the comparators74 and 76 shown and described with respect to FIG. 5. In other words,the circuit of FIG. 3 produces a cut-in and a cut-out voltage as afunction of the difference between the actual and the desiredtemperature in area 30 to respectively replace the voltage generated bypotentiometers 75 and 77 in the control circuit shown in FIG. 5. In thismanner, the two fixed voltages, the cut-in and cut-out pressures,normally generated by potentiometers 75 and 77, are now adjustable inreponse to the integrated difference between the desired and the actualtemperature in the coldest compartment to be cooled.

As previously described, the integrator 48 acts as a memory device, itremembers what has happened in the past. If the compartment 30temperature is below the desired norm, the integrator 48 will increaseits output voltage at a rate controlled by the temperature difference,i.e., the voltage difference. A greater difference produces a fasterchange. This increased voltage increases both the cut-in and cut-outvoltages to the comparators 74 and 76. These increased pressure limitsresult in an increase in the average controlled suction line pressure.

The rate at which this suction line pressure seeks out the new higherpressure is controlled in part by the difference between the desired andthe actual temperatures in the coldest compartment. An increased suctionline pressure results in less cooling from coil 27 and a warming of area30. When the temperature in area 30 reaches the desired setting, therewill no longer be any voltage into integrator 48, and the signal, TEMP.INT. INPUT will stabilize at a higher than previous value. Thisintegrator process eventually results in an optimum suction linepressure operating condition, i,e., the highest suction line operatingpressure possible to satisfy the cooling demands of the coldestcompartment. A higher suction line pressure equates directly to a lowercompressor capacity requirement which, in turn, produces a reduction inpower consumption to run the refrigeration system.

Referring now to the circuit shown in FIG. 3, the signal TEMP. INT.INPUT is shown coupled through a forward conducting diode 87 into avoltage setting network of resistors 86 and 89 operating in combinationwith a pair of Zener diodes 82 and 84 connected in series arrangement.The Zener diode arrangement of diodes 82 and 84 is connected across thepotentiometer 86 thereby to set a constant voltage drop thereacross. Thevoltage from the high side of the potentiometer 86 is the pressurecontrol voltage for the CUT-IN COMPARATOR while the wiper voltage ofpotentiometer 86 is the pressure control voltage for the CUT-OUTCOMPARATOR. The low side of the potentiometer is connected to circuitground through an appropriate resistor 89. Because of the voltagedivider effect of the variable potentiometer 86 and the constant voltagedrop of Zener diodes 82 and 84 thereacross, the voltage of the CUT-OUTCOMPARATOR signal will always differ by a constant value from thevoltage of the CUT-IN COMPARATOR signal.

The voltage CUT-IN COMPARATOR will be controlled by the summation of thevoltage from potentiometer 80 and the output of integrator 48, TEMP.INT. INPUT, applied to the summing junction through diode 87. Thus, asthe output of integrator 48 increases, the suction line cut-in andcut-out pressure control voltages will likewise increase indicating thatthe normal operating suction line pressure will have to increase inorder to bring the temperature of the coldest compartment back to thenorm setting. Eventually, the control circuit 36 selects the appropriatecompressor capacity to achieve an average desired suction line operatingpressure which will satisfy the cooling requirement of the coldestcompartment. Because of the pressure regulator valves included in theevaporator coils of the other evaporator coils in the multipleevaporator coil system plus the fact that each of the other areas orcompartments to be cooled by the other evaporator coils are at a warmertemperature, the resulting average suction line pressure will beadequate to achieve the desired cooling of those compartmentalizedareas.

It has been discovered that operation of the current apparatus achievesapproximately a 20 percent reduction in energy consumption over the bestknown prior art system.

While a particular embodiment of the present invention has been shownand described, it will be understood that the invention is not limitedthereto, since many modifications may be made and will become apparentto those skilled in the art. For example, the above system has describedtemperature operation in conjunction with one coil or area which isdesirably operated colder than another coil or area. If desired, it ispossible to utilize a thermoelectric sensing means in conjunction withboth coils or areas in a two-coil or two-area system. An average outputor other suitable combined output can then be employed as the variableor operating output to integrator 48.

What is claimed is:
 1. A compressor controller circuit for use in amultiple compressor cooling system, said controller circuit having acontrol means responsive to the operating suction line pressure forcycling said compressors on and off in a sequence to select acombination of said compressors to provide the minimum compressorcapacity sufficient to obtain an average operating suction line pressurein the range defined from an upper to a lower pressure limit, saidcontrol means including a comparator means for generating increase anddecrease capacity signals when the suction pressure is, respectively,above said upper pressure limit and below said lower pressure limit,said control means selectively energizing and deenergizing saidcompressors by applying said increase and decrease capacity signals tosaid compressors in a sequence where the first energized of theenergized compressors is the next to be deenergized and the firstdeenergized of the deenergized compressors is the next to be energized,said controller further including a means responsive to the temperatureof an area cooled by a system evaporator coil for dynamically adjustingsaid upper and lower pressure limits, said means for adjusting saidupper and lower pressure limits including an integrator means responsiveto a desired operating set point area temperature and the areatemperature for integrating the difference therebetween, the integrateddifference adjusting said pressure limits thereby to obtain acombination of said compressors having the minimum system compressorcapacity required to obtained the highest average suction line pressurewhich will maintain said area temperature at a desired level.
 2. Thecontroller circuit of claim 1 wherein said means for adjusting saidupper and lower pressure limits further includes:(a) a temperaturesensor for measuring the temperature of said area; (b) a means forgenerating a signal representative of a desired operating temperaturefor the area; and (c) a reference voltage circuit responsive to theintegrated difference signal from said integrator means for generatingfirst and second reference voltages respectively representative of saidupper and lower pressure limits, said second reference voltage differingfrom said first reference voltage by a fixed amount, said control meansincluding means responsive to said suction line pressure and said firstand second reference voltage to increase and decrease said compressorcapacity when said suction line pressure is, respectively, greater thansaid upper pressure limit and said suction line pressure is less thansaid lower pressure limit, said integrator and said reference voltagecircuit cooperating together to raise and lower said pressure rangethereby to obtain the minimum compressor capacity required to operatewith the highest average suction line pressure in said range ofpressures which will maintain the desired area temperature.
 3. Thecontroller circuit of claim 2 wherein said comparator means includes:(a)a pressure sensor for measuring the suction line operating pressure; (b)a first comparator responsive to said upper pressure limit voltage andto the output from said pressure sensor for generating an increasecapacity signal when the measured pressure is greater than said upperlimit; and (c) a second comparator responsive to said lower pressurelimit voltage and to the output from said pressure sensor for generatinga decrease capacity signal when the measured pressure is less than saidlower limit.
 4. The controler circuit of claims 1, 2 or 3 wherein saidupper and lower pressure limits are the same pressure.
 5. A compressorcontroller circuit for controlling the compressor capacity in a multiplecompressor cooling system, said cooling system including an evaporatorcoil for cooling an area to be cooled, said controller circuitcomprising:(a) a pressure selecting means for establishing an operatingsuction line pressure range having an upper limit and a lower limit; (b)a detection means for sensing the suction line pressure and cooperatingwith said pressure selecting means for determining when said suctionpressure exceeds said upper limit and providing an increase capacitysignal in response thereto, and when said suction pressure is below saidlower limit and providing a decreased capacity signal in responsethereto; (c) a sequencing means connected to said detection means forestablishing a first-off first-on sequence for energizing thecompressors in response to increase capacity signals and forestablishing a first-on first-off sequence for deenergizing thecompressors in response to decrease capacity signals; (d) a controlcircuit means responsive to said sequencing means for energizing saidcompressors in a first-off first-on sequence, and for deenergizing saidcompressors in a first-on first-off sequence to select a combination ofsaid compressors to provide the compressor capacity sufficient to obtainan average operating suction line pressure in said suction line pressurerange defined from said upper to said lower suction pressure limits; and(e) a presure range control means responsive to the temperature of anarea cooled by a system evaporator coil for dynamically adjusting saidupper and lower suction pressure limits to obtain a combination ofenergized said compressors having the minimum system compressor capacityrequired to obtain the highest average suction line pressure which willmaintain said area temperature at a desired level.
 6. The controllercircuit of claim 5 wherein said pressure range control means foradjusting said upper and lower pressure limits includes:(a) atemperature sensor for measuring the temperature of said area; (b) ameans for generating a signal representative of a desired operatingtemperature for the area; and (c) an integrator responsive to saidmeasured area temperature and said desired operating temperature signalfor integrating the difference therebetween.
 7. The controller circuitof claim 6 wherein said pressure selecting means comprises:(a) areference voltage circuit for generating first and second referencevoltages respectively representative of said upper and lower pressurelimits; and (b) a comparator means responsive to said suction linepressure and said first and second reference voltages to generate saidincrease and decrease capacity signals when said suction line pressureis, respectively, greater than said upper pressure limit and saidsuction line pressure is less than said lower pressure limit, saidpressure range control means and said pressure selecting meanscooperating together to raise and lower said pressure range limitsthereby to obtain the minimum compressor capacity required to operatewith the highest average suction line pressure in said range ofpressures which will maintain the desired area temperature.
 8. Thecontroller circuit of claim 7 wherein said comparator means includes:(a)a first comparator responsive to said upper pressure limit voltage andto the output from said detection means for generating said increasecapacity signal when the measured pressure is greater than said upperlimit; and (b) a second comparator responsive to said lower pressurelimit voltage and to the output from said detection means for generatingsaid decrease capacity signal when the measured pressure is less thansaid lower limit, said control circuit means selectively energizing anddeenergizing said compressors by applying said increase and decreasecapacity signals to said compressors in a sequence where the firstenergized of the enerzied compressors is the next to be deenergized andthe first deenergized of the deenergized compressors in the next to bethe energized.
 9. Apparatus for controlling the capacity of apreselected number of commonly piped compressors in a refrigerationsystem to obtain a minimum average system compressor capacity sufficientto obtain an average operating suction line pressure, comprising:(a) apressure selecting means for establishing an operating suction pressurerange having an upper limit and a lower limit; (b) a detection means forsensing the suction pressure in the system and cooperating with saidpressure selecting means for determining when said pressure exceeds saidupper limit and providing an increase capacity signal in responsethereto, and when said pressure is below said lower limit and providinga decrease capacity signal in response thereto; (c) a selection meansfor receiving said increase capacity signals and said decrease capacitysignals and in respective response thereto, selectively energizing anddeenergizing said compressors to provide combinations of energizedcompressors that exceed in total possible numbers of combinations thenumber of compressors in the system so that the average compressorcapacity of the system produces an average suction line pressure in saidoperating suction pressure range; and (d) a suction pressure rangecontrol means responsive to the temperature of an area cooled by asystem evaporator coil for dynamically adjusting said upper and saidlower pressure limits to obtain the minimum average system compressorcapacity required to obtain the highest average suction line pressurewhich will maintain said area temperature at a desired level.
 10. Theapparatus of claim 9 wherein said suction pressure range control meansfor adjusting said upper and lower pressure limits includes:(a) atemperature sensor for measuring the temperature of said area; (b) ameans for generating a signal representative of a desired operatingtemperature for the area; and (c) an integrator responsive to saidmeasured area temperature and said desired operating temperature signalfor integrating the difference therebetween.
 11. The apparatus of claim10 wherein said pressure selecting means comprises:(a) a referencevoltage circuit for generating first and second reference voltagesrespectively representative of said variable upper and lower pressurelimits, said second reference voltage differing from said firstreference voltage by a fixed amount; and (b) a comparator meansresponsive of said suction line pressure and said first and secondreference voltages to increase and decrease said compressor capacitywhen said suction line pressure is, respectively, greater than saidupper pressure limit and said suction line pressure is less than saidlower pressure limit, said pressure range control means and saidpressure selecting means cooperating together to raise and lower saidpressure range thereby to obtain the minimum average compressor capacityrequired to operate with the highest average suction line pressure whichwill maintain the desired area temperature.
 12. The apparatus of claim10 wherein said comparator means includes:(a) a first comparatorresponsive to said variable upper pressure limit voltage and to theoutput from said detection means for generating an increase capacitysignal when the measured pressure is greater than said upper limit; and(b) a second comparator responsive to said variable lower pressure limitvoltage and to the output from said detection means for generating adecrease capacity signal when the measured pressure is less than saidlower limit.
 13. The apparatus of claim 12 wherein each said compressorhas a maximum cycle repetition rate, said system further including atime delay means responsive to said first and second comparators fordelaying said compressor turn on and turn off operations in response toreceipt of increase and decrease capacity signals, respectively.
 14. Theapparatus of claim 9, wherein said selection means comprises:(a) asequencing means for establishing a first-off first-on sequence forenergizing the compressors and a first-on first-off sequence fordeenergizing the compressors and receiving said increase and decreasecapacity signals for generating a compressor turn-on signal in responseto receiving said increase capacity signal and a compressor turn-offsignal in response to receiving said decrease capacity signal; and (b) acontrol circuit means for applying said turn-on signal to thedeenergized compressor stage that was the first to be deenergized andapplying said turn-off signal to the energized compressor stage that wasthe first to be energized.
 15. The system as described in claim 14,wherein said sequencing means comprises a counting means receiving saidincrease or decrease capacity signals from said detection means andgenerating in response to each of said received signals a cut-in orcut-out control signal to respectively turn on or turn off a compressorstage, said cut-in control signals occurring in a first-off first-onsequence and said cut-out control signals occurring in a first-onfirst-off sequence.
 16. A method of optimizing the system compressorcapacity in a multiple compressor cooling system to obtain the highestaverage operating suction line pressure which results in an evaporatorcoil maintaining the temperature of an area cooled by said coil to adesired temperature, comprising the steps of:(a) establishing anoperation suction pressure range having an upper limit and a lowerlimit; (b) detecting the suction pressure in the system for determiningwhen said suction pressure exceeds said upper limit and when saidsuction pressure is below said lower limit; (c) generating an increasecapacity signal when said operating suction pressure exceeds said upperlimit and a decrease capacity signal when said operation suctionpressure is below said lower limit; (d) applying said increase capacitysignal and said decrease capacity signal to respectively energize anddeenergize the preselected number of compressors in a sequence where thefirst deenergized compressor is the next to be energized and the firstenergized compressor is the next to be deenergized; (e) sensing thetemperature of said area; (f) integrating the difference between thesensed area temperature and a desired temperature; and (g) adjustingsaid suction pressure limits as a function of the integrated differenceobtained in step (f) to obtain the minimum compressor capacity whichwill produce the highest average operating suction line pressure tomaintain the area temperature at the desired temperature.
 17. Apparatusfor controlling the system compressor capacity for a cooling systemhaving a plurality of commonlypiped compressors coupled to a pluralityof evaporator coils and operated in accordance with control suction linepressure limits, comprising:(a) a temperature sensor for measuring thetemperature of the evaporator coil to be operated at the coldesttemperature; (b) a setpoint means for setting a desired operatingtemperature for the evaporator coil to be operated at the coldesttemperature; (c) a pressure level setting means responsive to saidmeasured evaporator temperature and said desired operating temperaturefor dynamically adjusting an upper and a lower suction line pressurelimit as a function of the difference between the desired temperatureand the measured temperature, said pressure level setting meansincluding,(i) an integrator responsive to said measured evaporatortemperature and said desired operating temperature for integrating thedifference therebetween, and (ii) a comparator reference voltagegenerator circuit responsive to the output voltage from said integratorfor generating first and second reference voltages respectivelyrepresentative of said upper and lower suction line pressure limits,said second reference voltage differing from said first referencevoltage by a fixed amount, said integrator and said reference voltagegenerator circuit cooperating together to increase said upper and lowerpressure limits when said measured temperature is less than said desiredtemperature, and to decrease said upper and lower pressure limits whensaid measured temperature is greater than said desired temperature; and(d) a compressor controller circuit responsive to said variable upperand lower suction line pressure limits for cycling said compressors onand off in a sequence to select a combination of said compressors whichproduces the minimum system compressor capacity required to obtain thehighest average operating suction line pressure between said variableupper and lower pressure limits which maintains the measured evaporatortemperature at the desired operating temperature, said evaporator coilto be maintained at the coldest temperature including a defrost meanshaving a lock-out switch coupled to said integrator for disabling saidintegrator when said lowest temperature evaporator coil switches todefrost operation.
 18. Apparatus for controlling the system compressorcapacity for a cooling system having a plurality of commonlypipedcompressors coupled to a plurality of evaporator coils, said apparatuscontrolling the compressor capacity in accordance with control suctionline pressure limits, comprising:(a) a temperature sensor for measuringthe temperature of the evaporator coil to be operated at the coldesttemperature; (b) a setpoint means for setting a desired operatingtemperature for the evaporator coil to be operated at the coldesttemperature; (c) a pressure level setting means responsive to saidmeasured evaporator temperature and said desired operating temperaturefor dynamically adjusting an upper and a lower suction line pressurelimit as a function of the difference between the desired temperatureand the measured temperature, said pressure level setting meansincluding,(i) an integrator responsive to said measured evaporatortemperature and said desired operating temperature for integrating thedifference therebetween, and (ii) a comparator reference voltagegenerator circuit responsive to the output voltage from said integratorfor generating first and second reference voltages respectivelyrepresentative of said upper and lower suction line pressure limits,said second reference voltage differing from said first referencevoltage by a fixed amount, said integrator and said reference voltagegenerator circuit cooperating together to increase said upper and lowerpressure limits when said measured temperature is less than said desiredtemperature, and to decrease said upper and lower pressure limits whensaid measured temperature is greater than said desired temperature; and(d) a compressor controller circuit responsive to said variable upperand lower suction line pressure limits for cycling said compressors onand off in a sequence to select a combination of said compressors whichproduces the minimum system compressor capacity required to obtain thehighest average operating suction line pressure between said variableupper and lower pressure limits which maintains the measured evaporatortemperature at the desired operating temperature, said compressorcontroller circuit including,(i) a pressure sensor for measuring thesuction line operating pressure, (ii) a first comparator responsive tosaid upper pressure limit voltage and to the output from said pressuresensor for generating an increase capacity signal when the measuredpressure is greater than said upper limit, and (iii) a second comparatorresponsive to said lower pressure limit voltage and to the output fromsaid pressure sensor for generating a decrease capacity signal when themeasured pressure is less than said lower limit, said control meansselectively energizing and deenergizing said compressors by applyingsaid increase and decrease capacity signals to said compressors in asequence where the first energized of the energized compressors is thenext to be deenergized and the first deenergized of the deenergizedcompressors is the next to be the energized.
 19. A refrigeration systemhaving in a closed loop connection a plurality of commonly pipedcompressors, each having an inlet and an outlet end and where thepressure at the inlet ends of said commonly piped compressors is thesuction line pressure for the system, a condenser connected to theoutlet end of said compressor and responsive to a high pressure gaseousphase recirculating refrigerant for condensing the refrigerant from itsgaseous to its liquid phase, at least one evaporator coil having aninlet and an outlet end and connected to said condenser and to saidcompressors, an expansion valve associated with each said evaporatorcoil and having an inlet and an outlet end connected between the outletend of said condenser and the inlet end of said associated evaporatorcoil, said condenser delivering high pressure liquid refrigerant to theinlet end of said expansion valve, said refrigerant expanding as itflows through said expansion valve, one said evaporator coil associatedwith an area to be cooled, and a compressor controller circuit forcontrolling the compressor capacity, said controller circuitcomprising:(a) a pressure selecting means for establishing an operatingsuction line pressure range having an upper limit and a lower limit; (b)a detection means for sensing the suction line pressure and cooperatingwith said pressure selecting means for determining when said suctionpressure exceeds said upper limit and providing an increase capacitysignal in response thereto, and when said suction pressure is below saidlower limit and providing a decrease capacity signal in responsethereto; (c) a sequencing means connected to said detection means forestablishing a first-off first-on sequence for energizing thecompressors in response to increase capacity signals and forestablishing a first-on first-off sequence for deenergizing thecompressors in response to decrease capacity signals; (d) a controlcircuit means responsive to said sequencing means for energizing saidcompressors in a first-off first-on sequence, and for deenergizing saidcompressors in a first-on first-off sequence to select a combination ofsaid compressors to provide the compressor capacity sufficient to obtainan average operating suction line pressure in said suction line pressurerange defined from said upper to said lower suction pressure limits; and(e) a pressure range control means responsive to the temperature of anarea cooled by a system evaporator coil for dynamically adjusting saidupper and lower suction pressure limits to obtain a combination ofenergized said compressors having the minimum system compressor capacityrequired to obtain the highest average suction line pressure which willmaintain said area temperature at a desired level.
 20. The refrigerationsystem of claim 19 wherein said pressure range control means foradjusting said upper and lower pressure limits includes:(a) atemperature sensor for measuring the temperature of said area; (b) ameans for generating a signal representative of a desired operatingtemperature for the area; and (c) an integrator responsive to saidmeasured area temperature and said desired operating temperature signalfor integrating the difference therebetween.
 21. The controller circuitof claim 19 wherein said pressure selecting means comprises:(a) areference voltage circuit for generating first and second referencevoltages respectively representative of said upper and lower pressurelimits; and (b) a comparator means responsive to said suction linepressure and said first and second reference voltages to generate saidincrease and decrease capacity signals when said suction line pressureis, respectively, greater than said upper pressure limit and saidsuction line pressure is less than said lower pressure limit, saidpressure range control means and said pressure selecting meanscooperating together to raise and lower said pressure range limitsthereby to obtain the minimum compressor capacity required to operatewith the highest average suction line pressure in said range ofpressures which will maintain the desired area temperature.
 22. Therefrigeration system of claim 21 wherein said comparator meansincludes:(a) a first comparator responsive to said upper pressure limitvoltage and to the output from said detection means for generating saidincrease capacity signal when the measured pressure is greater than saidupper limit; and (b) a second comparator responsive to said lowerpressure limit voltage and to the output from said detection means forgenerating said decrease capacity signal when the measured pressure isless than said lower limit, said control means selectively energizingand deenergizing said compressors by applying said increase and decreasecapacity signals to said compressors in a sequence where the firstenergized of the energized compressors is the next to be deenergized andthe first deenergized of the deenergized compressors is the next to bethe energized.
 23. A refrigeration system having in a closed loopconnection a plurality of commonly piped compressors, each having aninlet and an outlet end and where the pressure at the inlet ends of saidcommonly piped compressors is the suction line pressure for the system,a condenser connected to the outlet end of said compressor andresponsive to a high pressure gaseous phase recirculating refrigerantfor condensing the refrigerant from its gaseous to its liquid phase, atleast one evaporator coil having an inlet and an outlet end andconnected to said condenser and to said compressors, an expansion valveassociated with each said evaporator coil and having an inlet and anoutlet end connected between the outlet end of said condenser and theinlet end of said associated evaporator coil, said condenser deliveringhigh pressure liquid refrigerant to the inlet end of said expansionvalve, said refrigerant expanding as it flows through said expansionvalve, one said evaporator coil associated with an area to be cooled,and a compressor controller circuit for controlling the capacity of saidcommonly piped compressors to obtain a minimum average system compressorcapacity sufficient to obtain an average operating suction linepressure, comprising:(a) a pressure selecting means for establishing anoperating suction line pressure range having an upper limit and a lowerlimit; (b) a detection means for sensing the suction pressure in thesystem and cooperating with said pressure selecting means fordetermining when said pressure exceeds said upper limit and providing anincrease capacity signal in response thereto, and when said pressure isbelow said lower limit and providing a decreased capacity signal inresponse thereto; (c) a selection means for receiving said increasecapacity signals and said decrease capacity signals and in respectiveresponse thereto, selectively energizing and deenergizing saidcompressors to provide combinations of energized compressors that exceedin total possible numbers of combinations the number of compressors inthe system so that the average compressor capacity of the systemproduces an average suction line pressue in said operating suctionpressure range; and (d) a suction pressure range control meansresponsive to the temperature of an area cooled by a system evaporatorcoil for dynamically adjusting said upper and said lower pressure limitsto obtain the minimum average system compressor capacity required toobtain the highest average suction line pressure which will maintainsaid area temperature at a desired level.
 24. The refrigeration systemof claim 23 wherein said suction pressure range control means foradjusting said upper and lower pressure limits includes:(a) atemperature sensor for measuring the temperature of said area; (b) ameans for generating a signal representative of a desired operatingtemperature for the area; and (c) an integrator responsive to saidmeasured area temperature and said desired operating temperature signalfor integrating the difference therebetween.
 25. The refrigerationsystem of claim 23 wherein said pressure selecting means comprises:areference voltage circuit for generating first and second referencevoltages respectively representative of said variable upper and lowerpressure limits, said second reference voltage differing from said firstreference voltage by a fixed amount; and (b) a comparator meansresponsive to said suction line pressure and said first and secondreference voltages to increase and decrease said compressor capacitywhen said suction line pressure is, respectively, greater than saidupper pressure limit and said suction line pressure is less than saidlower pressure limit, said pressure range control means and saidpressure selecting means cooperating together to raise and lower saidpressure range thereby to obtain the minimum average compressor capacityrequired to operate with the highest average suction line pressure whichwill maintain the desired area temperature.
 26. The refrigeration systemof claim 25 wherein said means includes:(a) a first comparatorresponsive to said variable upper pressure limit voltage and to theoutput from said detection means for generating an increase capacitysignal when the measured pressure is greater than said upper limit; and(b) a second comparator responsive to said variable lower pressure limitvoltage and to the output from said detection means for generating adecrease capacity signal when the measured pressure is less than saidlower limit.
 27. The refrigeration system of claim 26 wherein each saidcompressor has a maximum cycle repetition rate, said system furtherincluding a time delay means responsive to said first and secondcomparators for delaying said compressor turn on and turn off operationsin response to receipt of increase and decrease capacity signals,respectively.
 28. The refrigeration system of claim 23, wherein saidselection means comprises:(a) a sequencing means for establishing afirst-off first-on sequence for energizing the compressors and afirst-on first-off sequence for deenergizing the compressors andreceiving said increase and decrease capacity signals for generating acompressor turn-on signal in response to receiving said increasecapacity signal and a compressor turn-off signal in response toreceiving said decrease capacity signal; and (b) a control circuit meansfor applying said turn-on signal to the deenergized compressor stagethat was the first to be deenergized and applying said turn-off signalto the energized compressor stage that was the first to be energized.29. The apparatus of claim 17 wherein said upper and lower suction linepressure limits comprise a single suction line control pressuresetpoint.
 30. The apparatus of claim 18 wherein said control suctionline pressure limits comprises a single suction line control pressurefor both said upper and lower pressure limits.
 31. The refrigerationsystem of claims 19 or 23 wherein said operating suction line pressurerange comprises a single suction line control pressure for both saidupper and lower pressure limits.